Owing to many advantageous characteristics, zinc-base plated steel sheets are widely used as various kinds of rust-preventive steel sheets. For using the zinc-base plated steel sheets as the rust-preventive steel sheets for automobiles, it is important for them to have corrosion resistance and coating adaptability, and further to have performance requested in the automobile body manufacturing process, including excellent press-formability, spot weldability, adhesiveness, and chemical conversion treatment performance.
The zinc-base plated steel sheets, however, generally have a drawback of poor press-formability compared with cold-rolled steel sheets. The inferiority is caused by higher sliding resistance between the zinc-base plated steel sheet and the press-die than that of the case of cold-rolled steel sheet. When the sliding resistance is high, the zinc-base plated steel sheet at sections near the bead portion becomes difficult in incoming into the press-die during pressing, which likely induces fracture of the steel sheet.
A method for applying high viscosity lubricant onto the zinc-base steel sheet is a common practice for improving the press-formability thereof. The method, however, raises problems of generation of coating defects in succeeding coating step caused by insufficient degreasing and of generation of unstable press performance caused by break of lubricant oil film. Consequently, the request for improving press-formability of zinc-base plated steel sheet is strong. Conventionally, the following-described technologies are presented to improve the press-formability of zinc-base plated steel sheet.
(1) JP-A-4-176878, (the term “JP-A” referred herein signifies the “Japanese Patent Laid-Open No.”), discloses a zinc-base plated steel sheet having a layer consisting mainly of an oxide and/or a hydroxide of one or more metals selected from the group consisting of Mn, Mo, Co, Ni, Ca, Cr, V, W, Ti, Al, and Zn, and a coating film consisting mainly of an oxygen acid of P and B and/or an oxide colloid of one or more elements selected from the group consisting of Si, Al, and Ti.
(2) JP-A-8-296058 discloses a method for manufacturing zinc-base plated steel sheet containing the steps of activating the surface of the zinc-base plated steel sheet, and of forming an inorganic oxide coating film containing one or more of elements selected from the group consisting of Mn, Mo, Co, Ni, Ca, V, W, P, and B.
(3) JP-A-9-170084 discloses a plated steel sheet having a plating layer on a zinc-base plated steel sheet and an amorphous product generated from a reaction between phosphorus and zinc on the plating layer, and a method for manufacturing the plated steel sheet.
(4) JP-A-4-88196 discloses a zinc-base plated steel sheet having excellent press-formability and chemical conversion treatment performance, containing an amorphous P oxide coating on the zinc-base plated steel sheet.
These technologies described above, however, have problems shown below.
The technology (1) conducts treatment of the zinc-base plating layer using an aqueous solution containing an etching assistant such as sulfuric acid and an oxidizing agent such as nitric acid ion and potassium permanganate. If that type of aqueous solution contacts with the zinc-base plating layer, the zinc in the plating ingredients dissolves in the aqueous solution, thus the zinc likely enters the formed coating film. As a result, the formed coating film secures the adhesiveness at the interface with the plating layer, which allows maintaining the function for covering the plating layer following the deformation of the plating layer. The technology, however, has problems described below. That is, since the coating film as described above covers the zinc-base plating layer, the reaction between the chemical conversion treatment liquid and the zinc cannot fully proceed during the chemical conversion treatment which is given as the pre-coating treatment for automobile, (the term “chemical conversion treatment” referred herein is a phosphate treatment, and is expressed as the “chemical conversion treatment” to differentiate from the treatment applied in the present invention), which induces problems such as generation of coarse crystals or fail in generating crystals. Generally, the chemical conversion treatment liquid contains fluorine ion or the like to improve the etchability of coating film. If, however, that kind of additive is not added to the chemical conversion treatment liquid or if degradation in etchability occurs caused by the presence of impurities, the above-described coating film cannot fully be dissolved or cannot be removed during the chemical conversion treatment, which results in significant appearance of the above-described phenomenon.
The technologies (2) through (4) also have similar problems as given above. That is, the technology (2) has features of increasing the reactivity of plating layer and increasing the bonding force between the plating layer and the inorganic oxide coating film formed on the plating layer. The technology (3) has a feature of forming an amorphous product of the reaction between phosphoric acid and zinc on the surface of the plating layer. The technology (4) has a feature of covering the plated steel surface with an amorphous P oxide which does not dissolve even during the degreasing step. Because of these features, these technologies are difficult to remove the coating film during the chemical conversion treatment under a chemical conversion treatment condition of inferior in etchability, and the insufficient chemical conversion likely occurs.
Furthermore, the technologies (1) through (4) apply the zinc etching to intake the zinc in to the coating film. Generally, when phosphoric acid ion and zinc ion coexist, insoluble phosphate crystals are likely formed. Accordingly, if the zinc-base plated steel sheet is brought into contact with an aqueous solution containing phosphoric acid and having etchability function to dissolve zinc, the zinc which is an ingredient of crystal is successively supplied from the plating layer so that, when nuclei of phosphate crystals are once formed, the crystals are easily grown. For a coating layer containing that type of crystals, these crystal ingredients are removed during press-forming to deposit between the steel sheet and the press-die to degrade the slidability, thus inducing die galling, and may resulting in fracture of the material.